1. Field of the Invention
The invention relates to advanced epoxy resin powders for coatings exhibiting superior stress/strain properties after curing to a thermoset condition, and having improved cure speed. These coating powders are useful in the protection of, e.g., steel reinforcing bar, metal coil stock and other metallic substrates which may require cold working after being coated.
2. Description of the Art
Coatings used to protect metals against corrosion are required to meet several important criteria. They must be durable so as to avoid damage to the coated product during transportation or storage and they must not craze or crack or otherwise fail when subjected to bending or other forms of distortion. Epoxy coatings for metals are well known in the art. They possess excellent abrasion and impact resistance and can be formulated to aggressively adhere to cleaned metal surfaces. Further, they have been found to survive most of the exacting conditions for corrosion protection coatings. However, until recently, even these suffered from insufficient flexibility. This resulted in the need for on-site repair, of gaps or voids in the coating, following installation of protected metal structures. While it has been possible to improve cured epoxy coating flexibility, using combinations of aromatic and aliphatic epoxides, these benefits have been offset by reduction in the rate of cure.
Commonly used epoxy protective coatings may be prepared by reacting diglycidyl ethers of dihydric phenols with dihydric phenols and subsequently curing them with appropriate curing agents. Coatings of this type are suitably durable for the majority of applications; however, they are likely to fail if coated metal substrates, such as metal plate, reinforcing rod or pipes are subjected to bending or other forms of distortion. Failure occurs when the coatings crack and provide a channel of access by water or other contaminants which attack the underlying metal. One solution for such stress failure by cracking has been the addition of an aliphatic diepoxide to the diglycidyl ether of the dihydric phenol.
U.S. Pat. No. 4,758,638 discloses advanced epoxy resin compositions comprising a mixture of diepoxides and at least one compound having two hydroxyl groups per molecule. The diepoxide mixture comprises from 6% to less than 50% of an aliphatic diepoxide and from about 50% to about 94% of an aromatic diepoxide.
European patent application EP 0325146 discloses a powder epoxy coating composition for metal reinforcing bars. In this case an advanced epoxy resin comprises an aromatic based epoxy resin, an aliphatic based epoxy resin and a dihydric phenol. The relative amounts of aromatic and aliphatic epoxy resin are derived from diepoxides. In this mixture of diepoxides, from 70% to 95% of the epoxide groups are provided by the aromatic diepoxide and from 30% to 5% of the epoxide groups are provided by the aliphatic diepoxide. Comparison between these compositions and ones wherein the epoxy groups are provided exclusively by aromatic epoxides indicates that cured coatings of the latter are susceptible to disbonding and cracking, especially at low temperature. These references suggest that compositions containing less than 5% aliphatic epoxide and in excess of 95% aromatic epoxide would be unsuitable as cured corrosion protective coatings for metal substrates. It is taught that such compositions would have a high probability of failure by cracking because of reduced flexibility.
Surprisingly, the current inventors have discovered an advanced epoxy resin with reduced amounts of aliphatic epoxide which provides cured corrosion protection coatings exhibiting not only desired flexibility but an increased rate of cure, allowing application at higher coating speeds. This increases the efficiency and lowers the cost of the coating operation.
Other references to anticorrosive coatings disclose reactions between epoxy resins and trihydric and tetrahydric alcohols (JP 1131284). Epoxy resin compositions disclosed in JP 75039691 refer to epoxy resin compositions consisting of a polyglycidyl ether of an aliphatic polyol and one or more aliphatic or alicyclic polycarboxylic acids or anhydrides. The resulting compositions have rubber-like elasticity, excellent cold, heat and weather resistance and are recommended for outdoor use. However, these compositions are non-friable, making them unsuitable for powder coating applications. Further, hydroxy terminated oligomers are not disclosed for use as curing agents.
The present inventors have discovered that an advanced epoxy powder composition may be prepared which does not have the inflexibility of prior art compositions. The improved coating conforms, without cracking, to metal substrates which have been bent or otherwise distorted.
Surprisingly, concentrations of aliphatic epoxide may be lowered to a level where, based upon prior art teachings, undesirable embrittlement of the epoxy coating would be expected to occur. Thus, with epoxy coating compositions of the present invention, it is possible to improve the curing speed without loss of flexibility.
Compositions of the current invention, while containing both aromatic and aliphatic epoxides, exhibit more rapid curing than prior art compositions which also rely on a combination of aromatic and aliphatic epoxides.
With increasing curing speed it is possible to increase the rate of throughput on the coating line where epoxy powder is applied to heated metal substrates such as rod or pipe. This is also accompanied by an increase in the volume of powder applied. Conventional epoxy powders such as those described above, have a problem when the rate of powder application is increased. The increase in speed results in an overspray condition wherein ultra-fine strands of molten epoxy resin are ejected from the surface of the hot, curing, corrosion protection coating. Sometimes referred to as cobwebbing, this condition causes blockage within the coating apparatus which results in increased down-time. It is therefore necessary to formulate powder coatings for minimum overspray.
Coatings of the present invention are extremely effective in reducing the overspray problem of cobwebbing. They have been shown to outperform prior art compositions such as those disclosed by the above references.